Porous solids are of great technological importance due to their ability to interact with gases and liquids not only at the surface, but throughout their bulk. While large pores can be produced and well controlled in a variety of materials, nanopores in the range of 2 nm and below (micropores, according to IUPAC classification) are usually achieved only in carbons or zeolites. During the past decades major efforts in the field of porous materials have been directed toward control of the size, shape and uniformity of the pores.
Highly crystallized zeolites have a narrow pore size distribution, but discrete pore sizes and the fine-tuning of pore size is impossible in zeolites because pores are controlled by a lattice structure. Porous carbons produced by thermal decomposition of organic materials may have pore diameters down to 0.3 nm, or mesopores of several nanometers, but they typically have a broad pore size distribution that limits their ability to separate molecules of different sizes. Materials with a tunable pore structure at the atomic level and a narrow pore size distribution do not exist.
Selective etching of carbides is an attractive technique for the synthesis of various carbon structures from nanotubes to diamonds (Derycke et al. 2002 Nano Lett. 2:1043-1046; Gogotsi et al. 2001 Nature 411:283-287). Carbon produced by the extraction of metals from carbides is called carbide-derived carbon (CDC) (Gogotsi, et al. 1994 Nature 367: 628-630; Gogotsi, et al. 1997 J. Mater. Chem. 7:1841-1848). Since the rigid metal carbide lattice is used as a template and the metal is extracted layer-by-layer, atomic level control can be achieved during synthesis and the carbon structure can be templated by the carbide structure. Further structure modification and control can be achieved by varying the temperature, gas composition, and other process variables. Reaction (1):SiC+2Cl2(g)=SiCl4(g)+Chas been used for the production of silicon tetrachloride since 1918, but the remaining carbon was usually burned. The linear reaction kinetics of reaction (1) allows transformations to large depth, until the particle or component is completely converted to carbon (Ersoy et al. (2001) Mater. Res. Innovations 5:55-62). The transformation is conformal and does not lead to changes in sample size or shape.
During the last decades, various CDCs have been investigated and specific surface areas (SSA) of up to 2000 m2/g with small pore sizes have been reported (Gogotsi et al. 1997 J. Mater. Chem. 7:1841-1848; Boehm et al. Proc. 12th Biennial Conf. on Carbon 149-150 (Pergamon, Oxford, 1975); Gordeevet al. 2000 Phys. Solid State 42: 2314-2317; Fedorov et al. 1995 Russ. Chem. J. 39:73-83). Comparison of reported data on CDCs shows that, for different carbides (SiC, TiC, ZrC, B4C, TaC, and Mo2C) and chlorination temperatures, pores between 0.8 and 2.1 nm, determined by the structure of the carbide precursor and process parameters, were produced. However, no control over the pore size or distribution was disclosed.
The present invention provides a method of tuning pore size in CDCs by controlling the synthesis temperature.